This invention relates to salts of a short acting benzodiazepine, and to use of the salts as medicaments, in particular for sedative or hypnotic, anxiolytic, muscle relaxant, or anticonvulsant purposes.
European Patent No. 1,183,243 describes short-acting benzodiazepines that include a carboxylic acid ester moiety and are inactivated by non-specific tissue esterases. An organ-independent elimination mechanism is predicted to be characteristic of these benzodiazepines, providing a more predictable and reproducible pharmacodynamic profile. The compounds are suitable for therapeutic purposes, including sedative-hypnotic, anxiolytic, muscle relaxant and anticonvulsant purposes. The compounds are short-acting CNS depressants that are useful to be administered intravenously in the following clinical settings: preoperative sedation, anxiolysis, and amnestic use for perioperative events; conscious sedation during short diagnostic, operative or endoscopic procedures; as a component for the induction and maintenance of general anesthesia, prior and/or concomitant to the administration of other anaesthetic or analgesic agents; ICU sedation.
One of the compounds disclosed in EP 1,183,243 (in Example Ic-8, page 36) is Methyl 3-[(4S)-8-bromo-1-methyl-6-(2-pyridinyl)-4H-imidazol[1,2-a][1,4]benzodiazepin-4-yl]propanoate, as shown in formula (I) below:

Whilst the free base of formula (I) is stable when stored at 5° C., samples stored at 40° C./75% relative humidity (open) are observed to deliquesce, become yellow to orange in colour, and show notable decreases in content relative to initial (see Example 1 below).
It has now surprisingly been found that the compound of formula (I) forms highly crystalline mono (benzenesulfonic acid) besylate salts that are easily isolated from a range of pharmaceutically acceptable solvents and show good thermal stability, low hygroscopicity and high aqueous solubility.